1. Field Of The Invention
The present invention concerns a holder for a lead insert and a pin in a compass which is provided with a holder piece having a receiving member positioned in a cavity of the lower end portion of the flange, into which the lead insert or the needle can be arranged and tightened by means of a locknut, whereby the holder piece is provided with a groove extending into the receiving member which forms a channel nearly totally enveloping the lead insert or needle.
2. Description Of The Prior Art
German Utility Model No. 675570 discloses a holder for a lead insert and a needle which is open towards the base of the cavity of the lower end of the flange; during the tightening of the locknut resting on a threaded journal which sits somewhat in the central portion of the holder and which penetrates the base of the groove, the lead insert or needle is tightened between the rear wall of the holder borehole and the inner surface of the base of the groove. This structure has proven itself to be disadvantageous in many respects.
Thus, during the tightening of the locknut, there develops a bending of the holder in its center area, whereby the lead insert is also subjected to the bending. If this bending becomes too extensive, then the lead insert will break; however, when the locknut, in order to prevent such a stressing, is not pulled tight enough, then the lead insert is insufficiently secured and will fall out of the holder at the first chance. A secure clamping of the lead insert without dammaging the same is thus very difficult to obtain.
A further disadvantage is seen in the fact that, when the lead insert, due to useage, has sunk down below the threaded journal, the block, which supports itself in the upper area of the holder against the inner surface of the base of the groove, is missing. This leads to a tilting of the holder, and in such a manner that the lower end portion of the lead insert contacting the drafting paper is inwardly tiltable in the direction of the longitudinal center axis of the compass; there exists therefore the danger that the lead insert will rock or shake when in use.
Finally, it is a disadvantage that the borehole of the holder must be adapted to the largest-possible diameter of lead inserts or needles, since the borehole is not adjustable; this then results in a comparatively large play of the lead inserts or needles in the borehole when leads or needles with a fine crosscut have to be used. In such cases, there exists only a very unstable dual-line positioning of the lead inserts or needles.
German published Specification No. 19 03 653 discloses a needle-holder which is pivotably positioned at the lower fork-shaped end of a compass flange, wherein said holder is provided with a holding element having a receiver or borehole enveloping the needle almost entirely. The holder piece is provided with a longitudinal slot extending into the borehole and enabling the altering of the borehole crosscut and thereby its adaptation to different diameters of the needles; the plane of the slot forms a right angle with the axis of the threaded journal of the locknut.
Since the holding piece in this prior art structure--different than in the above-described holder in which the receiving of the lead insert or needle represents a groove enveloping the latter only partially--is provided with a channel enveloping the lead insert or needle almost entirely, there exists a planar contact between the lead insert or the needle on one hand and the holder piece on the other hand, which produces a favorable pressure distribution. Notwithstanding the fact that this prior art structure is suitable for receiving needles only, it, too, has not provided a solution to the existing needs.
Thus, it has proven a substantial disadvantage that an effective clamping of the needle in the borehole is obtainable only in that area of the holder piece where it is located between the flange fork formed at the lower end portion of the compass leg. Every place where the holder piece protrudes between these forked flanges, the tightening of the needle is reduced, so that there is obtained a variably strong tightening over the length of the holder piece. In the most unfavorable case, only that area which is adjacent the axial borehole receiving the threaded journal of the locknut is correctly tightened.